On Mon, Jan 09, 2017 at 11:24:04AM +0000, Robin Murphy wrote:

On 06/01/17 21:51, Andy Lutomirski wrote:

quoted

On Fri, Jan 6, 2017 at 10:32 AM, Robin Murphy [off-list ref] wrote:

quoted

On 06/01/17 17:48, Jean-Philippe Brucker wrote:

quoted

Hi Will,

On 20/12/16 15:14, Will Deacon wrote:

quoted

Booting Linux on an ARM fastmodel containing an SMMU emulation results
in an unexpected I/O page fault from the legacy virtio-blk PCI device:

[    1.211721] arm-smmu-v3 2b400000.smmu: event 0x10 received:
[    1.211800] arm-smmu-v3 2b400000.smmu:    0x00000000fffff010
[    1.211880] arm-smmu-v3 2b400000.smmu:    0x0000020800000000
[    1.211959] arm-smmu-v3 2b400000.smmu:    0x00000008fa081002
[    1.212075] arm-smmu-v3 2b400000.smmu:    0x0000000000000000
[    1.212155] arm-smmu-v3 2b400000.smmu: event 0x10 received:
[    1.212234] arm-smmu-v3 2b400000.smmu:    0x00000000fffff010
[    1.212314] arm-smmu-v3 2b400000.smmu:    0x0000020800000000
[    1.212394] arm-smmu-v3 2b400000.smmu:    0x00000008fa081000
[    1.212471] arm-smmu-v3 2b400000.smmu:    0x0000000000000000

<system hangs failing to read partition table>

This is because the virtio-blk is behind an SMMU, so we have consequently
swizzled its DMA ops and configured the SMMU to translate accesses. This
then requires the vring code to use the DMA API to establish translations,
otherwise all transactions will result in fatal faults and termination.

Given that ARM-based systems only see an SMMU if one is really present
(the topology is all described by firmware tables such as device-tree or
IORT), then we can safely use the DMA API for all virtio devices.

There is a problem with the platform block device on that same model.
Since it's not behind the SMMU, the DMA ops fall back to swiotlb, which
limits the number of mappings.

It used to work with 4.9, but since 9491ae4 ("mm: don't cap request size
based on read-ahead setting") unlocked read-ahead, we quickly run into
the limit of swiotlb and panic:

[    5.382359] virtio-mmio 1c130000.virtio_block: swiotlb buffer is full
(sz: 491520 bytes)
[    5.382452] virtio-mmio 1c130000.virtio_block: DMA: Out of SW-IOMMU
space for 491520 bytes
[    5.382531] Kernel panic - not syncing: DMA: Random memory could be
DMA written
...
[    5.383148] [<ffff0000083ad754>] swiotlb_map_page+0x194/0x1a0
[    5.383226] [<ffff000008096bb8>] __swiotlb_map_page+0x20/0x88
[    5.383320] [<ffff0000084bf738>] vring_map_one_sg.isra.1+0x70/0x88
[    5.383417] [<ffff0000084c04fc>] virtqueue_add_sgs+0x2ec/0x4e8
[    5.383505] [<ffff00000856d99c>] __virtblk_add_req+0x9c/0x1a8
...
[    5.384449] [<ffff0000081829c4>] ondemand_readahead+0xfc/0x2b8

Commit 9491ae4 caps the read-ahead request to a limit set by the backing
device. For virtio-blk, it is infinite (as set by the call to
blk_queue_max_hw_sectors in virtblk_probe).

I'm not sure how to fix this. Setting an arbitrary sector limit in the
virtio-blk driver seems unfair to other users. Maybe we should check if
the device is behind a hardware IOMMU before using the DMA API?

Hmm, this looks more like the virtio_block device simply has the wrong
DMA mask to begin with. For virtio-pci we set the streaming DMA mask to
64 bits - should a platform device not be similarly capable?

If it's not, then turning off DMA API will cause random corruption.
ISTM one way or another the bug is in either the DMA ops or in the
driver initialization.

OK, having looked a little deeper, I reckon virtio_mmio_probe() is
indeed missing a dma_set_mask() call compared to its PCI friends. The
only question then is where does virtio-mmio stand with respect to
legacy/modern/44-bit/64-bit etc.?

Robin.

AFAIK current drivers support the modern interface since Jan 2015.

44/64 is almost the same as PCI really, except page size isn't fixed to 4K.
So legacy ones need to set coherent mask to 32 + PAGE_SHIFT.

quoted

--Andy